For a 33 gallon tank, you're looking at 4-6 fish total, and mostly you'll want fish that are either bottom-dwelling or cave-dwelling. The more open-water fish are going to feel a bit stifled in a smaller tank, so you want fish that are happy with a limited territory and don't feel the need to swim a lot.
So with that as a general guideline, there are a few types of fish that are great for something in the 30 gallon range:
- Shrimp gobies (under 4-5" long)
- Most other gobies under 4" (avoid sand sifters other than Rainford/Hector)
- Firefish
- Clownfish (ocellaris/percula/skunk)
- Damsels (Chrysiptera genus)
- Blennies (under 4")
- Smaller basslets
- Some flasher wrasses
- Possum wrasses
- Some dwarf angels (Coral Beauty, Pygmy Cherub)
- Some hawkfish
For corals... size doesn't really matter, but flow, filtration and lighting all do. You can keep more or less whatever corals you want, but check for compatibility with the fish you want to keep. For example, if you want to keep acropora corals, I'd avoid clown gobies. Those gobies love to perch on acros. In a larger tank, their perching tends to get spread out among multiple corals and so it doesn't really impact much. In a smaller tank, they tend to overperch on just a few corals, and that can lead to tissue damage and death for the corals.
The one thing I would say to avoid is anemones for a 33 gallon tank. You can keep them just fine, that's not the issue. The issue is that anemones tend to split, and those splits can easily overwhelm a smaller tank.